1. Field of the Invention
The present invention relates to a polyurethane, and more particularly, to a thermoplastic polyurethane elastomer of a phase mixed system (homogeneous) exhibiting large electrostriction.
2. Description of the Background Art
There has been a strong interest in using electrostrictive polymers in applications such as sonars and transducers. It has recently been shown that the commercial polyurethane (for example under the trademark PELLETHANE by the Dow Chemical Company) thin films show a large electrostriction. The electrostrictive strain is greater than that calculated from Maxwell stresses however the mechanism responsible for this large electriction is not known. Electrostriction is a mechanical or elastic deformation caused by the application of an electric field to any dielectric material.
Polyurethane is a thermoplastic polymer (which can be made thermosetting) produced by the condensation reaction of a polyisocyanate and a hydroxyl-containing material, e.g., a polyol derived from propylene oxide or trichlorobutylene oxide. The basic polymer unit formed as follows:R1NCO+R2OH--->R1NHCOOR2 
High performance sonars are needed for use in shallow water for littoral use. These systems require a sonar driver material that demonstrates large strains at low frequencies. Recently, the commercial polymer, a polyurethane, has been shown to have an outstanding electrostrictive strain response two orders of magnitude greater than that of a ceramic or crystal. This large response along with a good impedance match to water indicates that this polyurethane is an excellent transducer material.
The chemical structure of the polyurethane molecule is a sequence of polar hard segment attached to a nonpolar hydrocarbon soft segment. The films of these polymers possess a morphology in which the highly polar hard segments separate and form regions embedded in a low polarity matrix of the soft segment. It has been postulated that the phase separation of the polar hard region was necessary for the large electrostriction to occur. One such mechanism was that the hard segments rotate due to the applied electric field and roll up the soft segment to cause a deformation. Another hypothesis was that the soft segment is elongated relative to a random coil due to its intrinsic molecular dissimilarity in polarization with the hard segment as well as additional stretching which occurs with hard segment phase separation. It was hypothesized that the application of the external electric field would release soft segment tie points to the hard segments and allow the soft segment to relax or contract to a random coil in the direction of the field. This was a plausible argument as the elongation of the soft segment was found by neutron scattering to be about 10% which is the order the macroscopic electrostrictive strain.
The art shows phase separated (heterogeneous) polyurethane system such as Dow 80AE PELLETHANE by the Dow Chemical Company. It is believed that phase separation between polar and nonpolar segments of the polyurethane chains is necessary for good electrostrictive properties.
Thermoplastic polyurethane elastomers can be used in transducers. Transducers perform energy conversions in one way by electrostrictive and piezoelectric fashion by using a ceramic or crystal, respectively, that establishes a voltage potential across one of its dimensions when one dimension of ceramic or crystal is changed in length due to compression or rarefaction. A voltage potential placed across the crystal or ceramic also causes a change in dimension.
Exemplars of the art are A Combined Experimental and Analytical Approach to the Measurement of Electrostrictive Coefficients on Polymers Films@ by Guillot et al., Science & Technology by McGraw-Hill Book Company; U.S. Pat. No. 5,229,979 to Scheinbeim et al., issued on Jul. 20, 1993, U.S. Pat. No. 4,833,659 to Geil et al., issued on May 23, 1989, U.S. Pat. No. 4,712,037 to Verbeek et al., issued on Dec. 8, 1987, U.S. Pat. No. 4,707,294 to Mathew et al., issued on Nov. 17, 1987, U.S. Pat. No. 4,011,818 to Stosz Jr. et al., issued on Mar. 15, 1977, U.S. Pat. No. 5,166,573 to Brown, issued on Nov. 24, 1992, U.S. Pat. No. 4,576,882 to Davis et al., issued on Mar. 18, 1986, U.S. Pat. No. 4,654,279 to Bauer et al., issued on Mar. 31, 1987, U.S. Pat. No. 5,343,443 to Merewether, issued on Aug. 30, 1994, U.S. Pat. No. 4,795,935 to Fujii et al., issued on Jan. 3, 1989, U.S. Pat. No. 4,549,107 to Kaneko et al., issued on Oct. 22, 1985, U.S. Pat. No. 5,340,510 to Bowen, issued on Aug. 23, 1994; and U.S. Pat. No. 4,877,988 to McGinniss et al., issued on Oct. 31, 1989.